The invention relates to a press section for a paper-making machine in which a continuous felt web, called a felt, carries a web of paper that is to be dewatered through a nip defined between two press rolls. The continuous felt must be periodically replaced.
It is known to provide the press section with what is called a cantilever beam to facilitate mounting a new felt. One example of a cantilevered press section is shown in U.S. Pat. No. 3,547,776. One of the press rolls, in this case the lower roll, is inside the continuous loop of the felt. The press roll rests on a component of the drier frame that is located on the operating side of the machine, i.e. the side having the machine controls on it, and also on another drier-frame component on the power side, where the drive connections to the rolls may be found. Two cantilever beams extend from the operating-side component to the power-side component and from there to a connection outside the press section. This connection is usually a structural element, although it is not mentioned in the above U.S. patent. When this paper-making machine is ready to operate, the drier-frame component on the operating side rests, through removable intermediate pieces, on one of the bases of the machine. The power-side component also rests on one of the machine bases, although its supports cannot be removed.
As a preliminary to mounting a new felt, the drier-frame component on the operating side of the machine must be slightly lifted to remove the intermediate pieces. One or two pneumatically or hydraulically operated piston cylinder combinations, spring or the like type lifting devices, or other force generators are usually positioned between the machine base and the operating-side component to lift the latter. Once the operating side component has been lifted and the intermediate pieces removed, the piston cylinders must be evacuated, leaving the operating-side drier-frame component supported only by the cantilever beams. In this state, the new felt can be introduced into the press section. The operating-side ends of the cantilever beams tend to sag under the weight of the drier-frame component and the press roll resting on it, so that the operating-side drier-frame component will drop. Since the cantilever beam is rigidly attached to the operating-side drier-frame component, that frame component will assume a skewed position as the cantilever beam deforms. This puts a lot of stress on the bearings that support the press roll because the distance between the operating-side and power-side bearings of each roll, which is resting on the operating-side component of the drier frame, increases. This leads to the risk of an excessive axial force in the roll bearings.
The above noted design has other drawbacks. The severe deformation of the cantilever beams generates powerful forces in threaded connections, especially in the connections between the beams and the operating-side drier frame components. Several time-consuming operations are necessary to insert a new felt. The operating-side component must be lifted, the intermediate pieces removed, the component released, the new felt inserted, the component lifted again, the intermediate pieces replaced, and the component supported on them again.
Another known system of mounting a new felt, described in German OS No. 2,239,267, required fewer operations. The essential characteristics of this known design include a cantilever beam that is non-rigidly attached to the power side drier-frame component by an articulated joint. The end of the cantilever beam outside the press section is connected to the supporting structure by a drawbar. The operating-side end of the beam supports a bearing housing for a press roll and is connected with the machine frame through a removable intermediate piece, which pivots around a vertical axis. A continuous felt and a continuous plastic wire screen, called a wire, travel over the press roll, which in this case is an upper roll.
The only step preliminary to mounting a new plastic wire is to draw down the end of the cantilever beam that is outside the press section with a tensioning device or force generator and only until the operating-side end drier frame component releases the intermediate piece supporting it. The intermediate piece on the operating-side end of the cantilever beam is then removed, the wire is introduced, and the original state is rapidly restored. One drawback to this known design, however, is that the operating-side drier-frame component which is in this case one of the press-roll bearing housings, is rigidly connected to the cantilever beam. Since the cantilever beam is in principle deformed in exactly the same way as in the first mentioned example, the bearing housing will also assume a skewed position, as will the leading-roll bearing housings when mounted on the cantilever beam.
There are also other considerations. Only one of these belts, the plastic wire, is easy to introduce into the press section, in the form of an open loop. In contrast, the felt can be mounted in this way only over the press roll and some of the felt-leading or other felt supporting rolls. To install the felt into its final position, it has to be folded or pleated, because in its final position, it loops around other felt-leading rolls that are supported on longitudinal beams at the top of the press section. Many contemporary felts, however, are too thick to be thus folded or pleated. Also, the folding or pleating cannot be done in the wider paper-making machines. One makeshift approach is to remove the felt-leading or supporting rolls, mount the loop of felt, and remount the rolls inside the felt loop. This method takes a lot of time and is extremely difficult in wide machines.